Casting steel strip

ABSTRACT

A method of casting steel strip by introducing molten plain carbon steel on casting surfaces at least one casting roll with the molten steel having a free nitrogen content below 120 ppm and a free hydrogen content below about 6.5 ppm measured at atmospheric pressure. The free nitrogen content maybe below about 100 ppm or below about 85 ppm. The free hydrogen content maybe between 1.0 and 6.5 ppm at atmospheric pressure. Novel cast strip of plain carbon steel is produced having a strip thickness less than 5 mm or less than 2 mm by use of the method.

This application claims priority to and the benefit of U.S. provisionalpatent application No. 60/510,479 filed Oct. 10, 2003, the disclosure ofwhich is incorporated herein by reference.

BACKGROUND AND SUMMARY OF THE DISCLOSURE

This invention relates to the casting of steel strip. It has particularapplication for continuous casting of thin steel strip less than 5 mm inthickness in a roll caster.

In a roll caster, molten metal is cooled on casting surfaces of at leastone casting roll and formed in to thin cast strip. In roll casting witha twin roll caster, molten metal is introduced between a pair of counterrotated casting rolls that are cooled. Steel shells solidify on themoving casting surfaces and are brought together at a nip between thecasting rolls to produce a solidified sheet product delivered downwardlyfrom the nip. The term “nip” is used herein to refer to the generalregion in which the casting rolls are closest together. In any case, themolten metal is usually poured from a ladle into a smaller vessel, fromwhere it flow through a metal delivery system to distributive nozzleslocated generally above the casting surfaces of the casting rolls. Intwin roll casting, the molten metal is delivered between the castingrolls to form a casting pool of molten metal supported on the castingsurfaces of the rolls adjacent to the nip and extending along the lengthof the nip. Such casting pool is usually confined between side plates ordams held in sliding engagement adjacent to ends of the casting rolls,so as to dam the two ends of the casting pool.

When casting thin steel strip with a twin roll caster, the molten metalin the casting pool will generally be at a temperature of the order of1500° C. and above. It is therefore necessary to achieve very highcooling rates over the casting surfaces of the casting rolls. A highheat flux and extensive nucleation on initial solidification of themetal shells on the casting surfaces is needed to form the steel strip.U.S. Pat. No. 5,760,336 incorporated herein by reference describes howthe heat flux on initial solidification can be increased by adjustingthe steel melt chemistry such that a substantial portion of the metaloxides formed are liquid at the initial solidification temperature, andin turn, a substantially liquid layer formed at the interface betweenthe molten metal and each casting surface. As disclosed in U.S. Pat.Nos. 5,934,359 and 6,059,014 and International Application AU 99/00641,the disclosures of which are incorporated herein by reference,nucleation of the steel on initial solidification can be influenced bythe texture of the casting surface. In particular, InternationalApplication AU 99/00641 discloses that a random texture of peaks andtroughs in the casting surfaces can enhance initial solidification byproviding substantial nucleation sites distributed over the castingsurfaces.

Attention has been given in the past to the steel chemistry of the melt,particularly in the ladle metallurgy furnace before thin strip casting.We have given attention in the past to the oxide inclusions and theoxygen levels in the steel metal and their impact on the quality of thesteel strip produced. We have now found that the quality of the steelstrip and the production of the thin steel strip is also enhanced bycontrol of the hydrogen levels and nitrogen levels in the molten steel.Controlling hydrogen and nitrogen levels has in the past been thesubject of investigation in slab casting, but to our knowledge has notbeen a focus of attention in thin strip casting. For example, seeControl of Heat Removal in the Continuous Casting Mould, by P. Zasowskiand D. Sosinsky, 1990 Steelmaking Conference Proceedings, 253–259; andDetermination and Prediction of Water Vapor Solubilities in CaO—MgO—SiO₂ Slags, by D. Sosinsky, M. Maeda and A. Mclean, MetallurgicalTransactions, vol. 16b, 61–66 (March 1985).

Specifically we have found that by controlling the hydrogen and nitrogenlevels in the steel melt, with low levels of sulfur in the steel, plaincarbon steel strip having unique composition and production qualitiescan be produced by roll casting. There is provided a method of castingsteel strip comprising:

introducing molten plain carbon steel on casting surfaces of at leastone casting roll with the molten steel having a free nitrogen contentbelow about 120 ppm and a free hydrogen content below about 6.9 ppmmeasured at atmospheric pressure and such that the sum of partialpressure of nitrogen and partial pressure of hydrogen is no more than1.15 atmospheres;

forming a casting pool of molten metal on the eastine surfaces of thecasting rolls; and

solidifying the molten steel to form metal shells on the casting rollshaving nitrogen and hydrogen levels reflected by the content thereof inthe molten steel to form thin steel strip. The content of the freehydrogen may be below about 6.5 ppm, and sum of partial pressure ofnitrogen and partial pressure of hydrogen in the introduced molten metalmay be no more than 1.0 atmosphere.

The method of casting steel strip may be carried out by the stepscomprising the following:

assembling a pair of cooled casting rolls having a nip between them andconfining end closures adjacent to ends of the casting rolls;

introducing molten plain carbon steel between the pair of casting rollsto form a casting pool on casting surfaces of the casting rolls with theend closures confining the pool, with the molten steel having a freenitrogen content below about 120 ppm and a free hydrogen content belowabout 6.9 ppm measured at atmospheric pressure and such that the sum ofpartial pressure of nitrogen and partial pressure of hydrogen is no morethan 1.15 atmospheres; and

counter-rotating the casting rolls and solidifying the molten steel toform metal shells on the casting rolls having nitrogen and hydrogenlevels reflected by the content thereof in the molten steel to providefor the formation of thin steel strip; and

forming solidified thin steel strip through the nip between the castingrolls to produce a solidified steel strip delivered downwardly from thenip. The content of the free hydrogen may be below about 6.5 ppm, andsum of partial pressure of nitrogen and partial pressure of hydrogen inthe introduced molten metal may be no more than 1.0 atmosphere.

Alternatively, there is provided a method of casting steel stripcomprising:

introducing molten plain carbon steel on casting roll surfaces of atleast one casting roll having a free nitrogen content below about 100ppm and a free hydrogen content below about 6.9 ppm measured atatmospheric pressure and such that the sum of partial pressure ofnitrogen and partial pressure of hydrogen is no more than 1.15atmospheres;

forming a casting pool of molten metal on the casting surfaces of thecasting rolls; and

solidifying the molten steel to form metal shells on the casting rollshaving nitrogen and hydrogen levels reflected by the content thereof inthe molten steel to form thin steel strip. The content of the freehydrogen may be below about 6.5 ppm, and sum of partial pressure ofnitrogen and partial pressure of hydrogen in the introduced molten metalmay be no more than 1.0 atmosphere.

The method of casting steel strip may be carried out by the stepscomprising the following:

assembling a pair of cooled casting rolls having a nip between them andconfining end closures adjacent to ends of the casting rolls;

introducing molten plain carbon steel between the pair of casting rollsto form a casting pool on casting surfaces of the casting rolls with theend closures confining the pool, with the molten steel having a freenitrogen content below about 100 ppm and a free hydrogen content belowabout 6.9 ppm measured at atmospheric pressure and such that the sum ofpartial pressure of nitrogen and partial pressure of hydrogen is no morethan 1.15 atmospheres;

counter-rotating the casting rolls and solidifying the molten steel toform metal shells on the casting rolls having nitrogen and hydrogenlevels reflected by the content thereof in the molten steel to providefor the formation of thin steel strip; and

forming solidified thin steel strip through the nip between the castingrolls to produce a solidified steel strip delivered downwardly from thenip. The content of the free hydrogen may be below about 6.5 ppm, andsum of partial pressure of nitrogen and partial pressure of hydrogen inthe introduced molten metal may be no more than 1.0 atmosphere.

As a further alternative, there is provided a method of casting steelstrip comprising:

introducing molten plain carbon steel on casting surfaces of at leastone casting roll with the molten steel having a free nitrogen contentbelow about 85 ppm and a free hydrogen content below about 6.9 ppmmeasured at atmospheric pressure and such that the sum of partialpressure of nitrogen and partial pressure of hydrogen is no more than1.15 atmospheres;

forming a casting pool of molten metal on the casting surfaces of thecasting rolls; and

solidifying the molten steel to form metal shells on the casting rollshaving nitrogen and hydrogen levels reflected by the content thereof inthe molten steel to form thin steel strip. The content of the freehydrogen may be below about 6.5 ppm, and sum of partial pressure ofnitrogen and partial pressure of hydrogen in the introduced molten metalmay be no more than 1.0 atmosphere.

The method of casting steel strip may be carried out by the stepscomprising the following:

assembling a pair of cooled casting rolls having a nip between them andconfining end closures adjacent to ends of the casting rolls;

introducing molten plain carbon steel between the pair of casting rollsto form a casting pool on the casting surfaces of the casting rolls withthe end closure confining the pool, with the molten steel having a freenitrogen content below about 85 ppm and a free hydrogen content belowabout 6.9 ppm measured at atmospheric pressure and such that the sum ofpartial pressure of nitrogen and partial pressure of hydrogen is no morethan 1.15 atmospheres;

counter-rotating the casting rolls and solidifying the molten steel toform metal shells on the casting rolls having nitrogen and hydrogenlevels reflected by the content thereof in the molten steel to providefor the formation of thin steel strip; and

forming solidified thin steel strip through the nip between the castingrolls to produce a solidified steel strip delivered downwardly from thenip. The content of the free hydrogen may be below about 6.5 ppm, andsum of partial pressure of nitrogen and partial pressure of hydrogen inthe introduced molten metal may be no more than 1.0 atmosphere.

In any of these methods, the free nitrogen content may be 60 ppm orless, and the free hydrogen content may be 1.0 to 6.5 ppm. The freehydrogen content may, for example, be between 2.0 and 6.5 ppm or between3.0 and 6.5 ppm.

Plain carbon steel for purpose of the present invention is defined asless than 0.65% carbon, less than 2.5% silicon, less than 0.5% chromium,less than 2.0% manganese, less than 0.5% nickel, less than 0.25%molybdenum and less than 1.0% aluminum, together with of other elementssuch as sulfur, oxygen and phosphorus which normally occur in makingcarbon steel by electric arc furnace. Low carbon steel may be used inthese methods having a carbon content in the range 0.001% to 0.1% byweight, a manganese content in the range 0.01% to 2.0% by weight, and asilicon content in the range 0.01% to 2.5% by weight, and low carboncast strip may be made by the method. The steel may have an aluminumcontent of the order of 0.01% or less by weight. The aluminum may, forexample, be as little as 0.008% or less by weight. The molten steel maybe a silicon/manganese killed steel.

In these methods, the sulfur content of the steel may be 0.01% or less;and the sulfur content of the steel may be 0.007% by weight.

In these methods, the free nitrogen may be measured by optical emissionspectrometry, calibrated against the thermal conductivity method adescribed below. The free hydrogen levels may be determined by aHydrogen Direct Reading Immersed System (“Hydris”) unit, made by HereausElectronite.

The maximum allowable free nitrogen and free hydrogen levels may be fortotal pressure not to exceed 1.0 atmospheres. Higher pressures may beutilized in certain conditions, and the levels of free nitrogen and freehydrogen can be corresponding higher. For example, as explained below, aferrostatic head may be 1.15, causing the free nitrogen levels and freehydrogen levels to be higher as shown in FIG. 3. But for purposes of theparameters of the present methods, the free nitrogen and free hydrogenlevels are measured a 1.0 atmospheres even through the actual levels offree nitrogen and free hydrogen in the molten metal are higher when themethods are practiced with higher positive atmospheric pressure.

The present invention provides cast steel strip with unique propertiesthat are described by the methods by which it is made. This steel stripis plain carbon steel.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully explained, illustrativeresults of experimental work carried out to date will be described withreference to the accompanying drawings in which:

FIG. 1 is a diagrammatic side elevation view of an illustrative stripcaster;

FIG. 2 is an enlarged sectional view of a portion of the caster of FIG.1;

FIG. 3 is a graph showing allowable nitrogen levels and hydrogen levelsin low carbon steel for a cast steel strip.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate a twin roll continuous strip caster which hasbeen operated in accordance with the present invention. The followingdescription of the described embodiments is in the context of continuouscasting steel strip using a twin roll caster. The present invention isnot limited, however, to the use of twin roll casters and extends toother types of continuous strip casters.

FIG. 1 shows successive parts of an illustrative production line wherebysteel strip can be produced in accordance with the present invention.FIGS. 1 and 2 illustrate a twin roll caster denoted generally as 11which produces a cast steel strip 12 that passes in a transit path 10across a guide table 13 to a pinch roll stand 14 comprising pinch rolls14A. Immediately after exiting the pinch roll stand 14, the strip maypass into a hot rolling mill 16 comprising a pair of reduction rolls 16Aand backing rolls 16B by in which it is hot rolled to reduce itsthickness. The rolled strip passes onto a run-out table 17 on which itmay be cooled by convection by contact with water supplied via waterjets 18 (or other suitable means) and by radiation. In any event, therolled strip may then pass through a pinch roll stand 20 comprising apair of pinch rolls 20A and thence to a coiler 19. Final cooling (ifnecessary) of the strip takes place on the coiler.

As shown in FIG. 2, twin roll caster 11 comprises a main machine frame21 which supports a pair of cooled casting rolls 22 having a castingsurfaces 22A, assembled side-by-side with a nip between them. Moltenmetal of plain carbon steel may be supplied during a casting operationfrom a ladle (not shown) to a tundish 23, through a refractory shroud 24to a distributor 25 and thence through a metal delivery nozzle 26generally above the nip 27 between the casting rolls 22. The moltenmetal thus delivered to the nip 27 forms a pool 30 supported on thecasting roll surfaces 22A above the nip and this pool is confined at theends of the rolls by a pair of side closures, dams or plates 28, whichmay be positioned adjacent the ends of the rolls by a pair of thrusters(not shown) comprising hydraulic cylinder units (or other suitablemeans) connected to the side plate holders. The upper surface of pool 30(generally referred to as the “meniscus” level) may rise above the lowerend of the delivery nozzle so that the lower end of the delivery nozzleis immersed within this pool.

Casting rolls 22 are water cooled so that shells solidify on the movingcasting surfaces of the rolls. The shells are then brought together atthe nip 27 between the casting rolls sometimes with molten metal betweenthe shells, to produce the solidified strip 12 which is delivereddownwardly from the nip.

Frame 21 supports a casting roll carriage which is horizontally movablebetween an assembly station and a casting station.

Casting rolls 22 may be counter-rotated through drive shafts (not shown)driven by an electric, hydraulic or pneumatic motor and transmission.Rolls 22 have copper peripheral walls formed with a series oflongitudinally extending and circumferentially spaced water coolingpassages supplied with cooling water. The rolls may typically be about500 mm in diameter and up to about 2000 mm long in order to producestrip product of about 2000 mm wide.

Tundish 25 is of conventional construction. It is formed as a wide dishmade of a refractory material such as for example magnesium oxide (MgO).One side of the tundish receives molten metal from the ladle and isprovided with an overflow spout 24 and an emergency plug 25.

Delivery nozzle 26 is formed as an elongate body made of a refractorymaterial such as for example alumina graphite. Its lower part is taperedso as to converge inwardly and downwardly above the nip between castingrolls 22.

Nozzle 26 may have a series of horizontally spaced generally verticallyextending flow passages to produce a suitably low velocity discharge ofmolten metal throughout the width of the rolls and to deliver the moltenmetal between the rolls onto the roll surfaces where initialsolidification occurs. Alternatively, the nozzle may have a singlecontinuous slot outlet to deliver a low velocity curtain of molten metaldirectly into the nip between the rolls and/or the nozzle may beimmersed in the molten metal pool.

The pool is confined at the ends of the rolls by a pair of side closureplates 28 which are adjacent to and held against stepped ends of therolls when the roll carriage is at the casting station. Side closureplates 28 are illustratively made of a strong refractory material, forexample boron nitride, and have scalloped side edges to match thecurvature of the stepped ends of the rolls. The side plates can bemounted in plate holders which are movable at the casting station byactuation of a pair of hydraulic cylinder units (or other suitablemeans) to bring the side plates into engagement with the stepped ends ofthe casting rolls to form end closures for the molten pool of metalformed on the casting rolls during a casting operation.

The twin roll caster may be of the kind illustrated and described insome detail in, for example, U.S. Pat. Nos. 5,184,668; 5,277,243;5,488,988; and/or 5,934,359; U.S. patent application Ser. No.10/436,336; and International Patent Application PCT/AU93/00593, thedisclosures of which are incorporated herein by reference. Reference maybe made to those patents for appropriate constructional details butforms no part of the present invention.

Results of the control of the free nitrogen and hydrogen levels in thincast sheets of plain carbon steel are set out in Table 1 and in FIG. 3.As FIG. 3 shows, where the free nitrogen level was below about 85 ppmand the free hydrogen level was below about 6.5 ppm the thin cast stripproduced was of premium “cold-rolled” steel quality. The heat(s) wherethe free nitrogen or free hydrogen level were above about 85 ppm orabout 6.5 ppm, respectively, did not produce thin cast strip of premiumcold-rolled steel quality. We have found, however, that hydrogen levelis the significant parameter and the nitrogen level can be higher up to100 ppm or 120 ppm

The results shown in FIG. 3 are for plain carbon thin rolled steel.Table 1 sets forth the analysis of each of the heats shown on FIG. 3. Asseen from FIG. 3, the left-hand curve shown is based on calculated basisfor total pressure of partial nitrogen and partial hydrogen equal to 1.0atmosphere.

TABLE 1 Last LMF Chems Seq ID LMF C LMF Si LMF Mn LMF N LMF S LMF P LMFAl H, ppm  822* 0.0493 0.265 0.6266 0.0075 0.011 0.0112 0.0042 7.3 10190.049 0.282 0.6122 0.0055 0.012 0.0113 0.0009 7 1057* 0.0622 0.28180.4894 0.008 0.013 0.0102 0.0008 8.3 1060* 0.0541 0.2986 0.5642 0.00810.0084 0.0107 0.0012 7.3 1071* 0.0547 0.1939 0.5616 0.0056 0.0076 0.00880.0029 5.6 1074* 0.0504 0.2989 0.5531 0.0042 0.0087 0.0149 0.002 6.31078* 0.0598 0.3212 0.6165 0.0081 0.0092 0.0155 0.0018 6.5 1079 0.05720.3368 0.6122 0.0067 0.0095 0.0117 0.0014 8.9 1080* 0.0582 0.2508 0.56880.0087 0.0119 0.011 0.0017 7.3 1082* 0.0606 0.2777 0.5603 0.0084 0.00940.0131 0.0016 7.4 1087* 0.0568 0.2794 0.5981 0.0078 0.0067 0.0166 0.00198.4 1088* 0.0534 0.3077 0.6044 0.0081 0.0106 0.0155 0.0025 8.3 10910.0479 0.2262 0.5565 0.0084 0.0095 0.026 0.0024 9 1095 0.0448 0.23430.5963 0.007 0.0086 0.0072 0.0013 8.5 1098* 0.0567 0.3831 0.4559 0.0080.0119 0.0111 0.0017 7 1099* 0.0532 0.2718 0.5324 0.0071 0.0109 0.01290.0015 6.8 1100* 0.0533 0.2685 0.5658 0.0074 0.0088 0.0108 0.0022 7.71103* 0.0548 0.2997 0.6137 0.0071 0.0115 0.012 0.0016 7.1 1104* 0.0540.2799 0.6771 0.0067 0.008 0.0114 0.0024 7.4 1106* 0.047 0.3229 0.62810.0058 0.01 0.0104 0.0028 7.6 1110* 0.0434 0.3068 0.6848 0.0046 0.0060.0111 0.0014 4.4 1111 0.0414 0.3002 0.5669 0.005 0.0089 0.0163 0.00195.6 1113* 0.0289 0.0798 0.4376 0.0044 0.0053 0.0101 0.0182 4.6 11130.0416 0.2212 0.5914 0.0053 0.0067 0.0119 0.0025 6.2 1114* 0.0489 0.30340.5943 0.0055 0.0058 0.008 0.0017 3.9 1115* 0.0594 0.3404 0.6565 0.00530.0064 0.0129 0.0021 4.7 1116 0.0507 0.3725 0.6806 0.0062 0.0095 0.01230.0051 5 1117 0.0437 0.2258 0.563 0.0067 0.008 0.0121 0.0012 5 1118*0.0629 0.3149 0.633 0.0081 0.0086 0.0143 0.001 7.7 1120* 0.0486 0.29350.5384 0.0077 0.0063 0.0074 0.0048 7.7 1121* 0.0492 0.314 0.6371 0.00730.0093 0.0163 0.0012 7.9 1122* 0.0525 0.2639 0.5867 0.0085 0.011 0.01410.0009 7.5 1123 0.0578 0.3238 0.5966 0.0058 0.0082 0.0124 0.0023 5.21125* 0.0682 0.3221 0.5786 0.0063 0.0055 0.0083 0.0005 4.7 1128* 0.04080.2456 0.5895 0.005 0.0083 0.0095 0.0016 5.1 1130 0.0378 0.3219 0.6270.0073 0.0087 0.0172 0.0023 5.1 1133 0.0398 0.2899 0.574 0.0054 0.00840.0092 0.0033 5.2 1134 0.0558 0.2612 0.6039 0.0055 0.009 0.0148 0.00385.9 1135 0.0567 0.2085 0.6093 0.0052 0.0125 0.0151 0.0015 4.6 1144*0.0554 0.3702 0.6315 0.0077 0.0098 0.0108 0.0027 6.7 1160* 0.0448 0.33380.5496 0.0054 0.0055 0.0078 0.004 4.4 1161 0.057 0.3182 0.6093 0.00540.0066 0.0092 0.0015 4.2 1163 0.0499 0.3198 0.6033 0.0053 0.0056 0.00780.0026 4.2 1164 0.0352 0.2783 0.59 0.0058 0.0058 0.0076 0.0025 3.6 11670.0451 0.3395 0.6026 0.0054 0.0073 0.0086 0.0024 3.5 1168 0.0515 0.28410.5897 0.0058 0.0043 0.0059 0.0018 3.9 1170 0.0366 0.2839 0.5958 0.00620.0054 0.0077 0.0018 4 1171 0.0454 0.304 0.586 0.007 0.0053 0.00730.0031 4.7 1172 0.0372 0.291 0.618 0.005 0.006 0.0087 0.0017 3.5 11730.0537 0.3049 0.6171 0.0051 0.0038 0.0086 0.0014 5.2 1180 0.054 0.27060.6285 0.0055 0.0069 0.006 0.001 4 1182 0.0543 0.3296 0.6386 0.00620.0082 0.0094 0.0013 4.5 1182 0.0511 0.3008 0.6025 0.0049 0.0057 0.00990.0015 4.2 1183 0.0549 0.2859 0.6147 0.0069 0.0082 0.0087 0.0003 3.71183 0.0492 0.2718 0.6245 0.0063 0.0054 0.0085 0.0007 3.8 1188 0.05110.3076 0.6298 0.0073 0.0042 0.0076 0.0048 4.4 1189 0.0562 0.3133 0.6460.0063 0.0031 0.0083 0.0085 3.2 1189 0.0452 0.3536 0.6902 0.0049 0.00140.0079 0.0132 4.1 1193* 0.0556 0.2864 0.6116 0.0059 0.0063 0.0084 0.00173.7 1196 0.0103 0.2989 0.6053 0.0052 0.0018 0.0082 0.0171 4 1198 0.05310.2643 0.6123 0.007 0.0064 0.0079 0.003 5 1200* 0.0534 0.2627 0.60820.0078 0.0107 0.007 0.0018 6.7 1205* 0.0544 0.2696 0.6037 0.0078 0.00970.0063 0.0011 6.8 *indicates reduced Heat Flux Sequences

The composition of all heats in Table 1 are in percent by weight, andare shown in FIG. 3. The heats were measured for a heat flux index of±0.7 megawatt per square meter from the desired level, i.e., range abouta standard heat flux for a given casting speed. Examples of standardheat flux for a given casting speed is 15 megawatts/m² for a castingspeed of 80 meters/min and 13 megawatts/m² for casting speed of 65meters/min. Astrerisk heats in Table 1 had the heat flux index within anacceptable range of ±0.7 megawatts pre square meter as shown in FIG. 3.The curve in FIG. 3 shows maximum allowable levels of free nitrogen andfree hydrogen for the summed partial pressures of the free nitrogen andfree hydrogen totaling 1.0 atmospheres to produce the acceptable heatflux index of ±0.7 megawatts per square meter. As shown in FIG. 3, allof the heats that had a free nitrogen level below about 85 ppm and afree hydrogen level below about 6.5 ppm had a heat flux within thedesired range except heats 1110 and 1125. In heat 1110, the free oxygenlevels were usually low, approximately 10 ppm, and in heat 1125, therewere mechanical problems in the casting equipment.

More recently, additional heats have been made with low nitrogen and lowhydrogen having compositions shown in Table 2. The nitrogen level rangefrom 42 to 118 ppm and the hydrogen levels ranged from 3.0 to 6.9 ppm.However, the hydrogen level of 6.9 ppm is with a ferrostatic head ofmore than 1 atmosphere pressure, namely about 1.15 atmospheres, as shownby the right-hand curve in FIG. 3.

TABLE 2 SEQ_ID HEAT_ID C MN N S SI P AL H, ppm 1734 248991 0.0502 0.56530.0042 0.0079 0.2615 0.0124 0.0006 4.6 1705 248296 0.048 0.5767 0.00540.0087 0.3154 0.017 0.0019 4.6 1701 142523 0.0461 0.5798 0.0053 0.00510.2729 0.0112 0.0008 5.1 1696 248237 0.0513 0.5793 0.0055 0.0052 0.29020.0112 0.0014 5 1695 248227 0.0559 0.5701 0.0066 0.0039 0.2436 0.01150.0006 6 1694 248207 0.0487 0.5763 0.0059 0.0081 0.2643 0.0172 0.00074.3 1691 248031 0.0481 0.5851 0.0063 0.0063 0.2605 0.0119 0.0006 4.41690 142250 0.0507 0.5928 0.0058 0.007 0.2582 0.0138 0.0009 3.2 1690142248 0.0554 0.5859 0.0079 0.0057 0.2583 0.017 0.001 4.3 1689 2480080.0473 0.5747 0.0051 0.0049 0.2631 0.014 0.0011 2.9 1689 248007 0.05380.575 0.0056 0.0055 0.2611 0.0127 0.0007 3.6 1688 248005 0.0493 0.58020.0053 0.0038 0.2629 0.0127 0.0008 4.6 1687 247994 0.0467 0.5974 0.00550.0045 0.2653 0.0129 0.001 3.8 1687 247992 0.0497 0.5791 0.0049 0.00560.2541 0.0114 0.0009 3.7 1684 247975 0.0498 0.5839 0.0061 0.0064 0.2480.012 0.0007 3.7 1684 247973 0.051 0.5716 0.0052 0.0031 0.2743 0.01220.0007 4.5 1683 247968 0.0488 0.5782 0.0062 0.0067 0.2774 0.0173 0.00083.9 1683 247965 0.0533 0.5753 0.0069 0.0081 0.2744 0.0183 0.0008 5 1681247954 0.0532 0.5354 0.0058 0.0061 0.2432 0.0152 0.0017 4.1 1680 2479340.0528 0.5861 0.0051 0.0049 0.2506 0.0106 0.0008 4.4 1679 247927 0.05240.5325 0.0063 0.0074 0.2521 0.0139 0.0007 4 1679 247925 0.0496 0.52660.0063 0.0065 0.2388 0.0121 0.0007 3.3 1679 247923 0.0549 0.5395 0.00630.0044 0.2354 0.0126 0.0007 4.5 1678 247917 0.0562 0.572 0.0052 0.00640.27 0.0156 0.0029 2.7 1678 247915 0.0499 0.6139 0.0052 0.0073 0.27890.0134 0.0009 3.3 1677 247910 0.0543 0.5721 0.0055 0.0088 0.2444 0.01630.0008 3.3 1677 247907 0.0491 0.5727 0.0076 0.008 0.2383 0.0214 0.00044.6 1676 142129 0.0505 0.5408 0.0061 0.0077 0.2374 0.0161 0.0005 3.91676 247898 0.0449 0.535 0.0052 0.0072 0.2589 0.0156 0.0008 3.9 1676247896 0.0521 0.54 0.0071 0.0051 0.2273 0.0139 0.0005 5.1 1675 2478940.0474 0.5398 0.006 0.0082 0.2442 0.0173 0.0005 3.3 1675 247892 0.04760.5845 0.0062 0.0092 0.2641 0.0215 0.0007 4.1 1674 247886 0.0518 0.60020.0061 0.0087 0.2544 0.0178 0.0022 3.3 1674 247884 0.0538 0.5682 0.00620.0081 0.2553 0.0164 0.0015 4 1673 142103 0.0471 0.5582 0.007 0.00630.2293 0.0207 0.003 4.1 1673 247874 0.0516 0.5262 0.0062 0.0049 0.24690.0161 0.0007 5.4 1672 247871 0.0533 0.5458 0.007 0.0057 0.2457 0.02160.0009 4.4 1672 247869 0.0478 0.554 0.0063 0.0059 0.2095 0.0242 0.00125.2 1671 247859 0.049 0.5848 0.0059 0.0051 0.2666 0.0108 0.0005 5 1670247848 0.0505 0.5728 0.0064 0.0062 0.2402 0.0207 0.0007 4.7 1667 2478170.0468 0.5921 0.0052 0.0059 0.268 0.0141 0.0013 3.5 1662 247612 0.04950.5773 0.0072 0.0075 0.2548 0.018 0.001 5.6 1657 247525 0.048 0.570.0068 0.004 0.257 0.019 0 4.8 1657 247524 0.051 0.58 0.0077 0.004 0.2460.016 0 5.8 1656 247515 0.0491 0.5768 0.0052 0.0076 0.2457 0.0115 0.00073.3 1656 247513 0.0496 0.5965 0.0053 0.0064 0.2916 0.0092 0.0008 4.21655 247507 0.0463 0.5777 0.0058 0.0093 0.2608 0.0117 0.0005 4.3 1655247505 0.0503 0.5691 0.0053 0.0061 0.2403 0.0173 0.0008 6.9 1654 2474900.0541 0.5753 0.0065 0.0064 0.2533 0.0094 0.001 4.2 1652 247484 0.04960.5877 0.0064 0.0064 0.251 0.0139 0.0009 5.3 1651 141683 0.0566 0.60040.0058 0.0061 0.2698 0.0094 0.0008 4.7 1650 247461 0.0467 0.5729 0.0060.0038 0.2663 0.0095 0.001 4.2 1650 141675 0.0519 0.5787 0.006 0.00520.2629 0.0098 0.0013 5 1649 141666 0.0546 0.6045 0.0056 0.0065 0.27550.0108 0.0009 4.2 1648 247441 0.0502 0.5949 0.0057 0.0049 0.2708 0.00970.0008 3.4 1648 247439 0.0493 0.5818 0.0047 0.0079 0.2588 0.012 0.00084.2 1647 247430 0.0483 0.5972 0.006 0.0037 0.2643 0.0069 0.0012 4.2 1646141641 0.0497 0.5954 0.0044 0.0054 0.3043 0.0062 0.0011 3.6 1645 2474100.0482 0.5731 0.0051 0.008 0.2456 0.0083 0.0007 3.8 1644 247403 0.050.6043 0.0065 0.0053 0.2547 0.0073 0.0007 4.2 1643 247399 0.0536 0.58010.0061 0.0054 0.2433 0.0075 0.0012 4.9 1642 247392 0.0531 0.5978 0.0050.0056 0.2651 0.009 0.001 3.5 1642 247390 0.0499 0.5788 0.005 0.00660.2669 0.0077 0.0008 3.1 1640 247377 0.0519 0.5601 0.0055 0.0085 0.25110.0099 0.0026 3.7 1639 247362 0.0507 0.5192 0.0069 0.0054 0.2132 0.00960.0005 3.7 1639 247360 0.0492 0.5146 0.006 0.0058 0.1896 0.0094 0.00044.5 1638 247352 0.0492 0.587 0.0065 0.0084 0.2734 0.009 0.0006 3.7 1638141578 0.0517 0.5727 0.0067 0.0111 0.2632 0.0155 0.0006 4.5 1637 2473370.0484 0.5415 0.0059 0.0069 0.2201 0.0115 0.0006 4.4 1637 247335 0.05310.5491 0.0068 0.0076 0.2374 0.0102 0.0009 4.5 1636 141557 0.0504 0.55920.0076 0.0087 0.2491 0.0114 0.0005 4.4 1634 247319 0.049 0.5424 0.00710.007 0.2094 0.0111 0.0003 4.6 1633 247310 0.0486 0.59 0.006 0.00890.2655 0.0098 0.0002 4.1 1632 247133 0.0519 0.5795 0.0067 0.005 0.25110.0093 0.0006 3.9 1632 247130 0.0461 0.5733 0.0058 0.0043 0.2421 0.00910.0004 4 1631 141348 0.0505 0.575 0.0057 0.0047 0.2434 0.0087 0.0007 3.51631 141347 0.0463 0.5886 0.0056 0.0065 0.2798 0.0098 0.0006 3.9 1630341342 0.0521 0.5775 0.0075 0.0077 0.2387 0.0133 0.0005 4.6 1624 1413000.0456 0.5921 0.005 0.0068 0.2586 0.0086 0.0006 4 1623 141288 0.0510.5978 0.0055 0.0064 0.2766 0.0107 0.0012 3.5 1621 247048 0.047 0.56130.0043 0.0066 0.2423 0.0112 0.0005 3.5 1621 247046 0.0499 0.553 0.00480.0062 0.2546 0.0105 0.0006 3.9 1620 247036 0.0531 0.5953 0.0053 0.00870.2463 0.0104 0.0008 3.5 1619 141253 0.0506 0.5932 0.005 0.007 0.25890.0152 0.0011 3.6 1619 141252 0.0485 0.5782 0.0064 0.0085 0.2363 0.01330.001 3.9 1618 247018 0.0532 0.589 0.0057 0.0077 0.2359 0.0104 0.00044.3 1617 247011 0.0457 0.5767 0.0051 0.0053 0.2647 0.0105 0.001 3.3 1616246997 0.0521 0.6192 0.0118 0.0044 0.2344 0.0072 0.0007 3.3 1611 2469570.0533 0.574 0.0076 0.0078 0.2251 0.0151 0.0004 4.2 1610 246942 0.04690.5853 0.0063 0.0085 0.2698 0.011 0.0007 3.3 1610 246940 0.0535 0.59260.0063 0.0081 0.2533 0.0093 0.0006 4 1609 141146 0.0529 0.5733 0.00540.0073 0.223 0.0101 0.0007 3.4 1609 141141 0.0547 0.5534 0.0069 0.0090.2169 0.0093 0.0005 4 1608 246915 0.0489 0.5895 0.006 0.007 0.27510.0093 0.0008 3.4 1607 141117 0.0537 0.5756 0.007 0.0077 0.2419 0.01220.0007 3.4 1606 141097 0.0512 0.5936 0.0057 0.0065 0.2582 0.0115 0.00053.6 1605 246877 0.0527 0.6154 0.0078 0.0056 0.2507 0.0092 0.0009 3.51605 246879 0.0497 0.5939 0.0055 0.0072 0.2418 0.0124 0.0009 3.1 1604246862 0.0483 0.6336 0.0053 0.006 0.2694 0.0088 0.001 4.6 1603 2468540.0522 0.6157 0.0058 0.0069 0.2587 0.0103 0.0011 3.2 1603 246852 0.05360.5455 0.005 0.0057 0.2468 0.01 0.0011 3.8 1602 246836 0.0468 0.60490.0044 0.0062 0.2748 0.0109 0.001 4.6 1601 246824 0.052 0.5846 0.00440.0103 0.2392 0.0126 0.0004 4.8 1598 246806 0.0459 0.5803 0.0041 0.0060.2684 0.0086 0.0006 4.4 1598 246804 0.0499 0.5795 0.0053 0.0077 0.26090.011 0.0005 5.2 1597 141011 0.044 0.5661 0.0061 0.0063 0.2635 0.01250.0006 5.3 1596 246777 0.0492 0.5378 0.0072 0.0052 0.2417 0.0115 0.00034.5 1595 140990 0.0428 0.5817 0.0053 0.0036 0.2529 0.0131 0.0009 4.31595 140988 0.0494 0.5583 0.0072 0.0071 0.2074 0.0107 0.0004 4.6 1594246759 0.048 0.5355 0.0064 0.009 0.2218 0.0094 0.0005 5.1 1594 1409780.0479 0.5645 0.0065 0.0068 0.228 0.0157 0.0005 5.6 1593 140976 0.05410.5799 0.0066 0.0074 0.2485 0.0143 0.001 4.5 1592 246741 0.047 0.56520.0053 0.0055 0.2348 0.0127 0.0009 4.9 1591 246739 0.0549 0.5755 0.00750.0041 0.2343 0.016 0.001 4.6 1590 246725 0.0404 0.575 0.0045 0.00790.2505 0.0109 0.0002 4 1589 140941 0.0524 0.5793 0.0053 0.0057 0.24140.0127 0.0011 4.9 1588 246565 0.0477 0.6328 0.0078 0.0065 0.2361 0.01660.0012 4 1587 246559 0.0457 0.5635 0.0055 0.0055 0.2446 0.0218 0.00023.8 1586 246546 0.0573 0.5793 0.0059 0.0094 0.2237 0.0134 0.0003 3.41585 246544 0.0601 0.5434 0.007 0.0067 0.2672 0.0198 0.001 3.5 1584246536 0.0538 0.5664 0.0064 0.0061 0.2087 0.0161 0.0008 3.8 1584 2465280.0488 0.559 0.0061 0.0051 0.2251 0.0166 0.0009 4.8 1583 246527 0.05190.5723 0.0067 0.0082 0.2173 0.0123 0.0007 4 1582 246520 0.0485 0.5820.0058 0.0108 0.2435 0.0137 0.0008 3.6 1582 246518 0.052 0.5639 0.00680.0104 0.2441 0.0121 0.0005 3.8 1579 246481 0.0514 0.5968 0.0063 0.00580.2555 0.0135 0.0007 3.3 1577 246459 0.0496 0.5945 0.0055 0.0056 0.25380.017 0.0005 3.1 1577 246457 0.0488 0.5943 0.006 0.0044 0.249 0.01560.0007 3.4 1576 246445 0.0446 0.549 0.0054 0.0031 0.2429 0.0105 0.00033.1 1575 246439 0.0498 0.5975 0.0049 0.0054 0.2644 0.0142 0.0006 3.21573 246414 0.0514 0.606 0.0047 0.0081 0.2639 0.0108 0.0005 3.2 1573246412 0.0475 0.5915 0.0043 0.006 0.2657 0.0144 0.0006 3.8 1572 2463930.0475 0.5955 0.0061 0.0072 0.2398 0.0113 0.0005 4.3 1570 246382 0.05010.5498 0.006 0.0071 0.2495 0.0122 0.0005 4.3 1569 246367 0.0563 0.57630.006 0.0064 0.2326 0.0108 0.0006 3.4 1569 246365 0.0501 0.5745 0.0060.0063 0.229 0.0127 0.0003 3.6 1568 246356 0.0486 0.5478 0.0058 0.00820.2374 0.0129 0.0026 3 1568 246354 0.0499 0.5564 0.0062 0.0078 0.24370.013 0.0013 3.3 1567 246341 0.0489 0.5659 0.006 0.0083 0.2291 0.01530.0002 3.3 1567 140568 0.0469 0.539 0.0061 0.0069 0.2159 0.0137 0.00043.5 1566 246331 0.0452 0.5614 0.0051 0.0086 0.2491 0.0129 0 2.7 1566246329 0.0433 0.5522 0.0054 0.0072 0.2514 0.0124 0.0006 3.4 1565 2463180.0504 0.5674 0.0047 0.0068 0.241 0.0115 0 3.8 1564 246304 0.0483 0.57080.0038 0.0077 0.2519 0.0119 0 3.1 1564 246302 0.0502 0.5742 0.005 0.00730.2563 0.0121 0.0002 3.5 1563 140529 0.0537 0.582 0.0066 0.0061 0.25740.0131 0 3.6 1561 140516 0.0546 0.5888 0.0048 0.006 0.2504 0.014 0 3.71561 246272 0.0495 0.5774 0.0051 0.0051 0.2423 0.0142 0 3.9 1560 1405020.0497 0.5865 0.005 0.0061 0.2626 0.0122 0.0004 3.2 1560 140500 0.04940.5902 0.0051 0.0037 0.2591 0.0154 0.0001 3.9 1558 246242 0.0479 0.60950.005 0.005 0.2586 0.0127 0.0006 3.9 1558 246240 0.0472 0.5867 0.00520.008 0.245 0.0107 0.0004 4.5 1556 246020 0.0522 0.607 0.0062 0.00770.2674 0.0085 0.0006 3.6 1555 140256 0.0554 0.5559 0.0061 0.0059 0.25040.0107 0.0003 4.3 1551 245974 0.0539 0.5876 0.0077 0.0064 0.2776 0.01280 4 1550 245965 0.0556 0.5781 0.0078 0.0054 0.2545 0.0127 0 3.9 1550245963 0.0513 0.5759 0.0074 0.0057 0.2686 0.0131 0 4 1549 245948 0.05490.5936 0.0075 0.0069 0.2493 0.0118 0.0002 3.6 1548 245938 0.0528 0.60590.0064 0.0059 0.273 0.0142 0.0002 3.7 1548 245936 0.0525 0.602 0.00670.0051 0.2828 0.0145 0.0001 3.7 1547 245925 0.0516 0.585 0.0069 0.00610.2543 0.0163 0.0003 3.4 1547 445923 0.0593 0.5902 0.0087 0.0087 0.2440.0195 0.0004 3.6 1545 245912 0.0509 0.567 0.0061 0.0076 0.2583 0.01710.0004 3.9 1544 245900 0.0535 0.5995 0.0055 0.0085 0.2546 0.0124 0.00073.4 1544 245898 0.0468 0.5968 0.0058 0.0086 0.2499 0.0143 0.001 3.4 1543140119 0.0492 0.5673 0.0062 0.0081 0.2386 0.0093 0 4.9 1540 2458640.0518 0.5756 0.0054 0.009 0.2595 0.0163 0.0004 3.6 1540 245863 0.04990.569 0.0055 0.0087 0.2646 0.015 0.0002 3.9 1539 245850 0.0544 0.58640.005 0.0082 0.2566 0.0125 0.0005 3.7 1538 245837 0.0542 0.5554 0.00570.007 0.2291 0.012 0.0002 4 1537 245825 0.0522 0.5892 0.0052 0.00510.2694 0.0098 0.0005 2.9 1537 245824 0.0505 0.5761 0.006 0.0065 0.27780.0134 0.0004 3.4 1536 140056 0.0512 0.5926 0.0065 0.0087 0.2416 0.01250.0002 3.5 1536 245814 0.0578 0.5835 0.0064 0.0098 0.2492 0.0121 0.00023.7 1535 140039 0.0492 0.5748 0.0072 0.0088 0.2393 0.012 0.0003 3.8 1535245797 0.0507 0.5567 0.0075 0.0087 0.2404 0.0113 0.0003 4.1 1534 2457890.0504 0.5519 0.0047 0.0068 0.2903 0.017 0.0007 2.9 1534 245788 0.05210.5839 0.0062 0.0048 0.2573 0.0152 0.0007 3.9 1533 245772 0.0539 0.58580.0067 0.0087 0.2602 0.014 0.0004 3.2 1533 245771 0.0557 0.5708 0.00690.0085 0.258 0.0143 0.0008 4.1 1532 245769 0.0483 0.5726 0.0055 0.00730.2318 0.0143 0.0001 3.6 1532 245767 0.0571 0.5644 0.0052 0.0059 0.23270.0137 0 3.8 1530 245559 0.0488 0.562 0.005 0.0043 0.2397 0.0191 0.00053.2 1529 245553 0.0541 0.6186 0.0072 0.009 0.2555 0.019 0.0004 3.7 1528245541 0.0507 0.5565 0.0066 0.0102 0.2477 0.0177 0.0003 3 1528 2455390.048 0.5393 0.0068 0.0096 0.2412 0.0178 0.0003 4.2 1527 245525 0.05570.5628 0.0062 0.0058 0.2499 0.0141 0.0004 3.6 1527 149763 0.0526 0.59410.0081 0.0072 0.2513 0.0154 0.0005 4.4 1522 245462 0.0456 0.6022 0.0050.0068 0.2665 0.0143 0.0006 2.9 1522 245461 0.0501 0.5844 0.0058 0.00770.2664 0.0153 0.0003 3.3 1521 149689 0.0478 0.6002 0.0054 0.0089 0.27970.0123 0.0005 3.6 1520 245443 0.0478 0.5367 0.0063 0.0064 0.2345 0.01730.0004 3.6 1517 245424 0.0541 0.5914 0.0071 0.0062 0.2368 0.0115 0.00033.7 1515 149635 0.051 0.6086 0.0064 0.0076 0.2751 0.0119 0.0004 3.5 1515149634 0.0549 0.6079 0.0065 0.0033 0.2653 0.0116 0.0004 3.5 1514 2454030.0491 0.5964 0.0071 0.0085 0.2261 0.0097 0.0001 3.5 1514 245400 0.0510.5616 0.0064 0.0087 0.2517 0.0109 0.0001 3.9 1513 149612 0.0448 0.58260.0057 0.0068 0.2585 0.0147 0.0004 3.2 1513 149610 0.0537 0.5647 0.00660.0082 0.2466 0.0136 0 3.5 1512 245373 0.051 0.5857 0.0058 0.0086 0.25120.0117 0.0005 2.8 1512 245371 0.0507 0.5571 0.0071 0.0075 0.2447 0.01170 4 1511 245353 0.0498 0.5823 0.0065 0.0063 0.2387 0.0109 0.0001 3.51510 245352 0.0532 0.5931 0.0065 0.0063 0.2623 0.0112 0.0001 3.8 1509245339 0.0504 0.564 0.0074 0.0089 0.2599 0.0137 0.0003 2.9 1508 2453330.0561 0.591 0.0071 0.0073 0.2541 0.0119 0.0003 3.6 1507 245308 0.05140.5784 0.0053 0.0046 0.2385 0.0118 0.0001 3.6 1506 245295 0.0456 0.58760.0053 0.005 0.2488 0.0095 0.0004 3.6 1506 245294 0.0521 0.6418 0.0060.0063 0.2718 0.0116 0.0005 2.9 1504 245287 0.0524 0.5863 0.0055 0.00420.2609 0.0127 0.0012 3.6 1503 245274 0.044 0.5684 0.0053 0.0068 0.25090.0096 0.0002 3.1 1503 149504 0.0485 0.5695 0.0057 0.0066 0.2449 0.00970.0002 3.5 1502 245262 0.0512 0.5974 0.004 0.0088 0.269 0.0091 0.00022.8 1502 245261 0.0475 0.579 0.0045 0.0068 0.256 0.0107 0.0008 4 1500245082 0.052 0.5876 0.0062 0.0106 0.2418 0.0107 0.0003 2.7

From the heats reported in Table 2, it is seen that the levels ofnitrogen can be up to 120 ppm, and the levels of hydrogen are between1.0, 2.0 or 3.0 and 6.5 ppm at atmospheric pressure. Moreover, thehydrogen level of 6.9 ppm in heat 1655 is with ferrostatic head of morethan 1 atmosphere pressure, namely about 1.15 atmospheres, as shown inFIG. 3.

The free nitrogen was determined by analysis with optical emissionspecometry (“OES”) calibrated against the thermal conductivity (“TC”)method on a scheduled basis. Optical emission spectrometry (OES) usingarc and spark excitation is the preferred method to determine thechemical composition of metallic samples. This process is widely used inthe metal making industries, including primary producers, foundries, diecasters and manufacturing. Due to its rapid analysis time and inherentaccuracy, Arc/Spark OES systems are most effective in controlling theprocessing of alloys. These spectrometers may be used for many aspectsof the production cycle including in-coming inspection of materials,metal processing, quality control of semi-finished and finished goodsand many other applications where a chemical composition of the metallicmaterial is required.

The Thermal Conductivity (TC) method, used to calibrate the OES,typically employs a microprocessor-based, software controlled instrumentthat can measure nitrogen, as well as oxygen, in a wide variety ofmetals, refractories and other inorganic materials. The TC methodemploys the inert gas fusion principle. A weighed sample, placed in ahigh purity graphite crucible, is fused under a flowing helium gasstream at temperatures sufficient to release oxygen, nitrogen andhydrogen. The oxygen in the sample, in all forms present, combines withthe carbon from the crucible to form carbon monoxide. The nitrogenpresent in the sample releases as molecular nitrogen and any hydrogen isreleased as hydrogen gas.

In the TC method, oxygen is measured by infrared absorption (IR). Samplegases first enter the IR module and pass through CO and CO₂ detectors.Oxygen present as either CO or CO₂ is detected. Following this, samplegas is passed through heated rare-earth copper oxide to convert CO toCO₂ and any hydrogen to water. Gases then re-enter the IR module andpass through a separate CO₂ detector for total oxygen measurement. Thisconfiguration maximizes performance and accuracy for both low and highrange.

In the TC method, nitrogen is measured by passing sample gases to bemeasured through heated rare-earth copper oxide which converts CO to CO₂and hydrogen to water. CO₂ and water are then removed to preventdetection by the TC cell. Gas flow then passes through the TC cell fornitrogen detection.

As stated above, the free hydrogen is measured by a Hydrogen DirectReading Immersed System (“Hydris”) unit, made by Hereaus Electronite.This unit is believed to be described in the following referenced USpatents: U.S. Pat. Nos. 4,998,432; 5,518,931 and 5,820,745.

While the invention has been illustrated and described in detail in theforegoing drawings and description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly illustrative embodiments thereof have been shown and described andthat all changes and modifications that come within the spirit of theinvention are desired to be protected. Additional features of theinvention will become apparent to those skilled in the art uponconsideration of the description. Modifications may be made withoutdeparting from the spirit and scope of the invention.

1. A method of casting steel strip comprising: introducing molten plaincarbon steel on casting surfaces of at least one casting roll with themolten steel having a free nitrogen content below about 120 ppm and afree hydrogen content below about 6.9 ppm and such that the sum ofpartial pressure of nitrogen and partial pressure of hydrogen is no morethan 1.15 atmospheres; forming a casting pool of molten metal on thecasting surfaces of the casting rolls; and solidifying the molten steelto form metal shells on the casting rolls having nitrogen and hydrogenlevels reflected by the content thereof in the molten steel to form thinsteel strip.
 2. The method of claim 1 where the free hydrogen content isbetween 1.0 and 6.5 ppm.
 3. The method of claim 1 where the sum ofpartial pressure of nitrogen and partial pressure of hydrogen in theintroduced molten metal is no more than 1.0 atmosphere.
 4. A method ofcasting steel strip comprising: assembling a pair of cooled castingrolls having a nip between them and confining end closures adjacent toends of the casting rolls; introducing molten plain carbon steel betweenthe pair of casting rolls to form a casting pool on the casting rollswith the end closures confining the pool, with the molten steel having afree nitrogen content below about 120 ppm and a free hydrogen contentbelow about 6.9 ppm and such that the sum of partial pressure ofnitrogen and partial pressure of hydrogen is no more than 1.15atmospheres; counter-rotating the casting rolls and solidifying themolten steel to form metal shells on casting surfaces of the castingrolls having nitrogen and hydrogen levels reflected by the content ofthe molten steel to provide for the formation of thin steel strip; andforming solidified thin steel strip through the nip between the castingrolls to produce a solidified steel strip delivered downwardly from thenip.
 5. The method of claim 4 where the sum of partial pressure ofnitrogen and partial pressure of hydrogen in the introduced molten metalis no more than 1.0 atmosphere.
 6. The method of claim 4 where the freehydrogen content is between 1.0 and 6.5 ppm.
 7. A method of castingsteel strip comprising: introducing molten plain carbon steel on castingsurfaces of at least one casting roll with the molten steel having afree nitrogen content below about 100 ppm and a free hydrogen contentbelow about 6.9 ppm and such that the sum of partial pressure ofnitrogen and partial pressure of hydrogen is no more than 1.15atmospheres; forming a casting pool of molten metal on the castingsurfaces of the casting rolls; and solidifying the molten steel to formmetal shells on the casting rolls having nitrogen and hydrogen levelsreflected by the content thereof in the molten steel to form thin steelstrip.
 8. The method of claim 7 where the free hydrogen content isbetween 1.0 and 6.5 ppm.
 9. A method of casting steel strip comprising:assembling a pair of cooled casting rolls having a nip between them andconfining end closures adjacent to ends of the casting rolls;introducing molten plain carbon steel between the pair of casting rollsto form a casting pool on the casting rolls with the end closuresconfining the pool, with the molten steel having a free nitrogen contentbelow about 100 ppm and a free hydrogen content below about 6.9 ppm andsuch that the sum of partial pressure of nitrogen and partial pressureof hydrogen is no more than 1.15 atmospheres; counter-rotating thecasting rolls and solidifying the molten steel to form metal shells oncasting surfaces of the casting rolls having nitrogen and hydrogenlevels reflected by the content of the molten steel to provide for theformation of thin steel strip; and forming solidified thin steel stripthrough the nip between the casting rolls to produce a solidified steelstrip delivered downwardly from the nip.
 10. The method of claim 9 wherethe free hydrogen content is between 1.0 and 6.5 ppm.
 11. The method ofclaim 9 where the sum of partial pressure of nitrogen and partialpressure of hydrogen in the introduced molten metal is no more than 1.0atmosphere.
 12. A method of casting steel strip comprising: introducingmolten plain carbon steel on casting surfaces of at least one castingroll with the molten steel having a free nitrogen content below about 85ppm and a free hydrogen content below about 6.9 ppm measured and suchthat the sum of partial pressure of nitrogen and partial pressure ofhydrogen is no more than 1.15 atmospheres; forming a casting pool ofmolten metal on the casting surfaces of the casting rolls; andsolidifying the molten steel to form metal shells on the casting rollshaving nitrogen and hydrogen levels reflected by the content thereof inthe molten steel to form thin steel strip.
 13. The method of claim 12where the free hydrogen content is between 1.0 and 6.5 ppm.
 14. A methodof casting steel strip comprising: assembling a pair of cooled castingrolls having a nip between them and confining end closures adjacent toends of the casting rolls; introducing molten plain carbon steel betweenthe pair of casting rolls to form a casting pool on the casting rollswith the end closures confining the pool, with the molten steel having afree nitrogen content below about 85 ppm and a free hydrogen contentbelow about 6.9 ppm and such that the sum of partial pressure ofnitrogen and partial pressure of hydrogen is no more than 1.15atmospheres; counter-rotating the casting rolls and solidifying themolten steel to form metal shells on casting surfaces of the castingrolls having nitrogen and hydrogen levels reflected by the content ofthe molten steel to provide for the formation of thin steel strip; andforming solidified thin steel strip through the nip between the castingrolls to produce a solidified steel strip delivered downwardly from thenip.
 15. The method of claim 13 where the free hydrogen content isbetween 1.0 and 6.5 ppm.
 16. The method of claim 12 where the sum ofpartial pressure of nitrogen and partial pressure of hydrogen in theintroduced molten metal is no more than 1.0 atmosphere.